Ink jet printers having one or more ink jet heads for projecting drops of ink onto paper or other printing medium to generate graphic images and text have become increasingly popular. To form color images, ink jet printers with multiple ink jet printing heads are used, with each head being supplied with ink of a different color. These colored inks are then applied, either alone or in combination, to the printing medium to make a finished color print. Typically, all of the colors needed to make the print are produced from combinations of cyan, magenta, and yellow ink. In addition, black ink may be utilized for printing textual material or for producing true four-color prints.
In a common arrangement, the print medium is attached to a rotating drum, with the ink jet heads being mounted on a traveling carriage that traverses the drum axially. As the heads scan paths over the medium, ink drops are projected from a minute orifice in each head to the medium so as to form an image on the medium. A suitable control system synchronizes the generation of ink drops with the rotating drum.
To produce images of certain colors, more than one color of ink is combined on the medium. That is, ink drops of a first color are applied to the medium and then overlaid by ink drops of a second color to produce the desired color of the image. If the drops do not converge on the same position on the medium, that is, if the drops of the two colors do not overlie one another, then the color of the image is distorted. Furthermore, it is also important that drops of substantially uniform size and shape be generated by the ink jet heads. To the extent that the drops are non-uniform, the image is distorted.
In one basic type of ink jet printing system, ink drops are produced on demand. An exemplary drop-on demand printer is illustrated in U.S. Pat. No. 4,106,032 of Miura et al. In the Miura printer, ink is delivered to an ink chamber in the ink jet head. Whenever a drop of ink is needed, an electric pulse is applied to a piezoelectric crystal, causing the crystal to constrict. As a result, because the crystal is in intimate mechanical contact with ink in the ink chamber, a pressure wave is transmitted through the ink chamber. This causes the formation of an ink drop at an internal drop-forming orifice outlet of the ink chamber. The ink drop passes from the drop forming orifice and through an air chamber toward a main external orifice of the ink jet head. This latter orifice leads to the printing medium. Air under pressure is delivered to the air chamber and entrains the drop of ink in a generally concentric air stream as the ink drop travels through the air chamber. This air stream increases the speed of the drops toward, and the accuracy of applying the drops to, the print medium.
Following the printing of a copy, in certain known ink jet printers, the ink jet heads are typically returned to a rest position against a mechanical cap until the next copy is printed. Also, when the power to such an ink jet printer is turned off, the air stream stops and these devices commonly flood the air chamber with ink from the ink chamber. Upon restarting the printer, the air stream is reestablished. It heretofore has been assumed that the reestablished air stream in the air chamber would blow all of the ink in the previously flooded air chamber outwardly from the main external orifice and into the cap.
These known devices suffer from a number of drawbacks. In particular, it is difficult to converge ink drops on particular positions of the print medium. Furthermore, it has been observed that the drop ejection angle shifts as these ink jet printers operate. The ink drop ejection angle is the angle between the line of travel of drops leaving the main external orifice of the ink jet print head and a line projecting from the main external orifice in a direction normal to the surface of the ink jet print head. This shifting further disturbs the convergence of the ink drops on the print medium. Moreover, with such devices, problems have been encountered in maintaining the uniformity of ink drop size and shape.
Therefore, a need exists for an improved method and apparatus for operating an ink jet head of an ink jet printer which is directed towards overcoming these and other disadvantages of prior art devices.